Flexible line trimmers are well-known and widely used devices for cutting soft vegetation. The cutting element is a length of flexible line, typically a monofilament, that is flailed against the vegetation by rapidly spinning the line with a motor. The line is strong enough to cut relatively soft vegetation, but it is not strong enough to cause serious injury to a person or to damage woody stemmed plants, trees or structures without breaking. Thus, flexible line trimmers tend to be safe to operate and are well suited for trimming grass and other vegetation adjacent to trees, sidewalks and houses, where a steel element or blade or other type of cutting element may cause damage or be damaged during normal use. Except for the fact that the line breaks frequently even during normal use, they are easy to use and tend to be relatively inexpensive and light-weight.
To overcome what is generally perceived to be the major deficiency of flexible line trimmers, a great deal of effort has been directed in the past to methods of making replenishing broken cutting line more convenient for users without complicating and increasing the cost of a line trimmer. The standard and currently most frequently employed approach is to sell a supply of replacement line on a spool and have the user mount it in a line head that is coupled to the motor of the line trimmer. A line head typically includes a hub, coupled to the output shaft of a motor, and a housing or shroud extending outwardly from the hub and down around the spool to protect the spool from dirt and debris. The user places the spool on the hub, feeding a small length of line through an opening in the housing, and places a cover over the spool to retain and protect the spool within the line head. The spool is locked to the hub so that rapid spinning of the line head fails the line. When line breaks, the spool is unlocked and spun relative to the hub to unwind line through the opening.
Several methods have been used to lock and unlock the spool against the hub. Early line heads used a manual release mechanism. To replace line, the trimmer was stopped and turned upside down. The locking mechanism was released manually and the spool turned to unwind and feed line. To avoid having to stop and turn the trimmer upside to feed line, most current line heads have a "bump-feed" mechanism that allows the user to feed a predetermined amount of line while the head is spinning by hitting a button on the bottom of the line head against the ground. More recently, line heads have features which automatically unlock the spool without any intervention of the user. Generally, these heads utilize the centrifugal force acting on the spinning head or flailing line to lock or to release the spool to rotate when the line becomes short or when the head accelerates upon start-up.
There are numerous examples of bump-feed mechanisms. Most of these mechanisms have an indexing means that permits the spool to rotate in the line head a predetermined amount relative to the hub or shroud when the line head is bumped against the ground. Bumping the button on the bottom releases a locking mechanism, allowing the spool to relatively rotate a predetermined angle on the hub as the whole hub assembly continues to rotate, and then locks the spool. The force to produce the relative rotation of the spool is provided by the centrifugal force of the portion of line outside the head. A predetermined length of line thus unwinds and is played out through the opening in the hub.
One type of indexing means includes a series of detentes between the spool and line head, usually formed by a series of tabs on the spool that cooperate with a series of stepped stops on either the outside surface of the hub or the inside surface of the housing. The spool is biased downwardly on the hub by a spring so that its tabs engage the lower set of stops on the hub or housing, thus locking the spool in a normal operating position. Bumping the line head against the ground displaces the spool axially upward to a second position so that its teeth clear the top of the series of stops. When the tapping action is completed, the spool is then free to rotate relative to the hub until the tabs on spool engage the stepped-up set of stops. The spring then forces the spool down where it continues to rotate on the hub until the tabs engage the lower set of stops.
Bump-feed line heads provide a great deal of convenience for the user. However, once a supply of flexible line on a spool is exhausted, the line trimmer must be turned on its side or upside down and held in an unwieldy position to replace the empty spool with another spool pre-wound with line and reattach the cover. A cover of a line head is often attached to the line head in a manner that demands such dexterity to unfasten and remove that many users become frustrated by the task of replacing a spool. The indexing mechanism of the line head unfortunately complicates and limits the choices of methods to retain spool in the line head. In one approach to make spool replacement easier, the spool is located in the handle of the trimmer, near the user. Examples of this configuration are illustrated in U.S. Pat. No. 4,369,577 of Gise et al. and U.S. Pat. No. 4,285,128 of Schnell et al. However, the line must then be fed through the handle shaft of the trimmer motor and then through line head, replacing one potentially frustrating task with another. U.S. Pat. No. 4,633,588 to Pittinger, Jr. retains the spool in the line head, but attempts to provide for an improved spool fastening mechanism that does not require removal of a cover. Instead, its fastening mechanism is released through a pin extending through the bottom of the spool. This pin-actuated release mechanism has, however, several disadvantages. It has several moving parts and is relatively complicated. It thus tends to be more expensive to manufacture and is more susceptible to sticking or interference from accumulation of dirt and debris around the pin and the fastening mechanism. The spool could thus be prematurely released during use or the pin become very difficult to push to release the spool.